Power amplifier system provided with improved protection function

ABSTRACT

A power amplifier system including a power terminal, a ground terminal, an output terminal, a ripple terminal, a control terminal to which a control signal is supplied from outside, a power amplifier circuit connected between the power terminal and the ground terminal, a negative potential detection circuit connected to the output terminal, and a bias circuit which supplies a bias voltage to the power amplifier circuit, and a bias start-up circuit controlling the startup operation of the bias circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/534,021 filed on Sep. 21, 2006, all of which is based upon and claimsthe benefit of priority from prior Japanese Patent Application No.2005-275997, filed on Sep. 22, 2005. The contents of each of thesedocuments are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power amplifier system for use in,for example, a car audio set, and more particularly, to a poweramplifier system provided with a protection function in case the GNDterminal is open and the output terminal is grounding (referred to as“GND open and output grounding”, hereinafter).

2. Description of the Related Art

FIG. 5 shows a block diagram of a power amplifier system that isinstalled in a car audio set. As shown in FIG. 5, a power amplifier IC50 includes, as terminals for external connection, a power terminal (Vccterminal) 11, a ground terminal (GND terminal) 12, an output terminal13, a ripple terminal 14, etc. Between the power terminal 11 and theground terminal 12 of the power amplifier IC 50, an externaldirect-current power supply (battery) 21 is connected, and also acontrol IC 22 having an external power IC, a microcomputer, etc. forstarting up the power amplifier IC 50 is connected (FIG. 5 shows thestate in which the battery is incorrectly connected, as will beexplained later). Furthermore, a ripple filter capacitor 23, or acapacitor 23 for a ripple filter, for smoothing a bias voltage isconnected between the ripple terminal 14 of the power amplifier IC 50and the ground, and a loudspeaker, not shown, is connected to the outputterminal 13. A control signal output from the control IC 22 is suppliedto a bias circuit 19.

In the power amplifier IC 50, a push-pull type power amplifier circuit16 is connected between the power terminal 11 and the ground terminal12. This push-pull type power amplifier circuit 16 is composed of a PMOStype push side output transistor PT and an NMOS type pull side outputtransistor NT, which are serially connected. The connection node ofthese transistors, that is, the push-pull output node of the poweramplifier circuit 16 is connected to the output terminal 13. In FIG. 5,“D” is a parasitic diode located between the drain and source of theoutput transistor NT connected to the ground potential side.Furthermore, these output transistors PT, NT have their gates connectedto a driver circuit 16 a.

A negative potential detection circuit 17 detects that the outputpotential of the power amplifier circuit 16 becomes negative, and thenegative potential detection output node thereof is connected to a gateimpedance control circuit 52.

The bias circuit 19 is controlled to be started up by a control signaloutput from the control IC 22. The bias circuit 19 supplies a biasvoltage to the driver circuit 16 a so as to make the power amplifiercircuit 16 perform operation, and the bias voltage output node thereofis connected to the driver circuit 16 a as well as to the rippleterminal 14.

The gate impedance control circuit 52 is connected between the gate andsource of the push side output transistor PT connected to the powersupply side, and is controlled by the negative potential detectioncircuit 17.

When the user installs thus configured power amplifier system in a caraudio set, there may be often raised errors in connecting or wiring thepower amplifier IC 50 and the battery 21. For example, as shown in FIG.5, in case the ground terminal of the battery 21 is incorrectlyconnected to the output terminal 13 of the power amplifier IC 50, theground terminal 12 of the power amplifier IC 50 is not connected to theground terminal of the battery 21 to come into open state (GND open andoutput grounding state). In this case, also the ground terminal of thecontrol IC 22 is not connected to the ground terminal of the battery 21.

In this incorrectly connected state, superficially, it is consideredthat the control IC 22 and the power amplifier IC 50 are not started upsince the ground terminal 12 thereof are open. However, as shown in FIG.5, since an operating current flows through the ground terminal 12,parasitic diode D, and output terminal 13 of the power amplifier IC 50,undesirably, the control IC 22 is started up correctly. That is, thecontrol IC 22 sends a control signal to start up the power amplifier IC50 without detecting the incorrect connection. Accordingly, regardlessof the incorrect connection, the bias circuit 19 of the power amplifierIC 50 is undesirably started up.

At the same time, the negative potential detection circuit 17 detectsthat the potential of the output terminal 13 of the power amplifier IC50 becomes negative, and the gate impedance control circuit 52 operatesdue to the detection output. At this time, as described above, since thebias circuit 19 is operating, it is determined whether or not the pushside output transistor PT can be protected depending on the operationstate of the gate impedance control circuit 52.

For example, in case the sensitivity of the gate impedance controlcircuit 52 is set higher and the gate impedance is suppressed to theminimum, the push side output transistor PT can be protected withoutbreakdown. However, in case the sensitivity of the gate impedancecontrol circuit 52 is set higher, when the power amplifier IC 50 iscorrectly connected to the power supply to drive loudspeakers, notshown, there may be raised a case in which the negative potentialdetection circuit 17 is made to malfunction due to the backelectromotive force of the loudspeakers. In this case, the push sideoutput transistor PT is made to cut off, and the sound quality is madedeteriorate.

In contradiction to the above, in case the sensitivity of the gateimpedance control circuit 52 is set lower and the gate impedance is madelarger, the bias circuit 19 is started up under the incorrect connectionstate. Thus, when the gate impedance control circuit 52 is operated bythe negative potential detection circuit 17, the push side outputtransistor PT cannot be protected sufficiently.

Accordingly, in the circuit shown in FIG. 5, so as to secure the soundquality when the power amplifier IC 50 is correctly connected as well asthe protection intensity of the push side output transistor PT at thetime of the GND open and when the output terminal is grounding, it isnecessary to set up the gate impedance control circuit 52 and thenegative potential detection circuit 17 carefully. However, sinceelements in a chip are subject to fluctuation depending on themanufacturing process, the design margin is extremely restricted withrespect to the design of respective circuits.

For this reason, there is desired a power amplifier system that cansurely prevent the breakdown of a push side output transistor withoutdeteriorating the sound quality of an output signal of a power amplifierIC even if a microcomputer etc. for controlling the power amplifiersystem sends a control signal for start-up operation to the poweramplifier IC when the power amplifier IC is incorrectly connected tocome into the GND open and output grounding state.

In the Jpn. Pat. Appln. Laid-Open Publication No. 2004-112019, there isdisclosed “grounding protection apparatus for sound power amplificationapparatus”. In this apparatus, a voltage detection unit detects anapplied power supply voltage from a power supply unit to be applied to asound power amplification circuit unit, and a voltage between unitground terminals. A control unit generates a switch drive signal to turnon a switch circuit when being judged to be appropriate, and turn offthe switch circuit when the sound power amplification circuit unit isjudged to be grounding, on the basis of the detection result of thevoltage detection unit.

In the Jpn. Pat. Appln. Laid-Open Publication No. 2001-7659, there isdisclosed “power amplifier”. This power amplifier is formed by a powertransistor circuit of the push-pull configuration whose last stage issupplied with a direct current bias. This power amplifier has builttherein a grounding breakdown prevention circuit for preventing the flowof an overcurrent by cutting off the direct current bias of a powertransistor when the loudspeaker drive output end comes into grounding.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a poweramplifier system comprising: a power terminal; a ground terminal; anoutput terminal; a ripple terminal; a control terminal to which acontrol signal for start-up is supplied from outside; first and secondMOS transistors that configure a power amplifier circuit connectedbetween the power terminal and the ground terminal, the first and secondMOS transistors configuring a push-pull circuit, the first and secondMOS transistors having the connection node thereof connected to theoutput terminal; a negative potential detection circuit being connectedto the output terminal, which detects the potential of the outputterminal becomes negative; a bias circuit which supplies a bias voltageto the power amplifier circuit, the bias circuit having its output nodefor outputting the bias voltage connected to the ripple terminal; a biasstart-up circuit to which the control signal supplied to the controlterminal, a detection signal supplied from the negative potentialdetection circuit, and the potential of the ripple terminal aresupplied, the bias start-up circuit controlling the start-up operationof the bias circuit in accordance with the control signal, the detectionsignal, and the potential of the ripple terminal; and a ripple filtercapacitor connected to the ripple terminal.

According to a second aspect of the invention, there is provided asemiconductor integrated circuit comprising: a power terminal; a groundterminal; an output terminal; a ripple terminal to which a ripple filtercapacitor is connected; a control terminal to which a control signal forstart-up is supplied from outside; first and second MOS transistors thatconfigure a power amplifier circuit connected between the power terminaland the ground terminal, the first and second MOS transistorsconfiguring a push-pull circuit, the first and second MOS transistorshaving the connection node thereof connected to the output terminal; anegative potential detection circuit being connected to the outputterminal, which detects the potential of the output terminal becomesnegative; a bias circuit which supplies a bias voltage to the poweramplifier circuit, the bias circuit having its output node foroutputting the bias voltage connected to the ripple terminal; and a biasstart-up circuit to which the control signal supplied to the controlterminal, a detection signal supplied from the negative potentialdetection circuit, and the potential of the ripple terminal aresupplied, the bias start-up circuit controlling the start-up operationof the bias circuit in accordance with the control signal, the detectionsignal, and the potential of the ripple terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of the first embodiment of a poweramplifier system according to the present invention;

FIG. 2 shows a circuit diagram of the first specific example of thenegative potential detection circuit shown in FIG. 1;

FIG. 3 shows a circuit diagram of the second specific example of thenegative potential detection circuit shown in FIG. 1;

FIG. 4 shows a circuit diagram of a specific example of the biasstart-up circuit, bias circuit, and ripple terminal potential detectioncircuit shown in FIG. 1; and

FIG. 5 shows a block diagram of a power amplifier system that isinstalled in a car audio set.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments according to the present invention will further bedescribed below with reference to the accompanying drawings. In theplural drawings, similar parts or components are indicated with the samereference numerals.

First Embodiment

FIG. 1 shows a block diagram of the first embodiment of a poweramplifier system according to the present invention. As shown in FIG. 1,a power amplifier IC 10 includes, as terminals for external connection,a power terminal (Vcc terminal) 11, a ground terminal (GND terminal) 12,an output terminal 13, a ripple terminal 14, a control terminal 15, etc.Between the power terminal 11 and the ground terminal 12 of the poweramplifier IC 10, an external direct-current power supply (battery) 21 isconnected, and also a control IC 22 having an external power IC, amicrocomputer, etc. is connected. Furthermore, a ripple filter capacitor23 for smoothing a bias voltage is connected between the ripple terminal14 of the power amplifier IC 10 and the ground, and a loudspeaker, notshown, are connected to the output terminal 13. A control signal outputfrom the external control IC 22, which operates using the battery 21 asthe operation power supply, is supplied to the control terminal 15 ofthe power amplifier IC 10.

In the power amplifier IC 10, a PMOS type push side output transistor PTand an NMOS type pull side output transistor NT are serially connectedbetween the power terminal 11 and the ground terminal 12. Thetransistors PT, NT configure a push-pull type power amplifier circuit16. An output node of the power amplifier circuit 16, that is, theconnection node of these output transistors PT, NT is connected to theoutput terminal 13. These push side output transistor PT and pull sideoutput transistor NT have their gates connected to a driver circuit 16a. In FIG. 1, “D” is a parasitic diode located between the drain andsource of the pull side output transistor NT connected to the groundpotential side.

A negative potential detection circuit 17 detects that the outputpotential of the power amplifier circuit 16 becomes negative, and thenegative potential detection output node thereof is connected to a biasstart-up circuit 18.

The bias start-up circuit 18 is controlled by a control signal suppliedfrom the control terminal 15, an output signal (negative potentialdetection signal) supplied from the negative potential detection circuit17, and the potential of the ripple terminal 14 to be described later,and supplies a bias start-up signal to a bias circuit 19 to be describedlater. The bias start-up circuit 18 is configured so that generation ofthe bias start-up signal is controlled in accordance with the logicallevel (active level/inactive level) of the control signal, negativepotential detection signal, and ripple potential detection signal, andthe combination of the change order thereof.

The bias voltage output node of the bias circuit 19 is connected to thedriver circuit 16 a as well as to the ripple terminal 14. The biascircuit 19 is started up by the bias start-up signal output from thebias start-up circuit 18, and supplies a bias voltage to the drivercircuit 16 a. Accordingly, the power amplifier circuit 16 is made toperform operation.

A ripple terminal potential detection circuit 20 detects the potentialof the ripple terminal 14. On the other hand, in case the potential ofthe ripple terminal 14 is directly utilized, the ripple terminalpotential detection circuit 20 can be omitted.

When the user installs thus configured power amplifier system in, forexample, a car audio set, there may be often raised errors in connectingor wiring the power amplifier IC 10 and the battery 21. For example, asshown by a dotted line EL in FIG. 1, in case the ground terminal of thebattery 21 is incorrectly connected to the output terminal 13 of thepower amplifier IC 10, the ground terminal 12 of the power amplifier IC10 is not connected to the ground terminal of the battery 21 to comeinto open state (GND open and output grounding state). In this case,also the ground terminal of the control IC 22, etc. is not connected tothe ground terminal of the battery 21.

In case of this incorrectly connected state, superficially, it isconsidered that the control IC 22 and the power amplifier IC 10 are notstarted up since the ground terminal thereof are open. However, since anoperating current flows through the ground terminal 12, parasitic diodeD, and output terminal 13 of the power amplifier IC 10, undesirably, thecontrol IC 22 is started up correctly. That is, the control IC 22supplies a control signal of active level to start up the poweramplifier IC 10 to the control terminal 15 of the power amplifier IC 10without detecting the incorrect connection. Accordingly, the controlsignal is supplied to the bias start-up circuit 18 through the controlterminal 15. At the same time, the negative potential detection circuit17 detects that the potential of the output terminal 13 of the poweramplifier IC 10 becomes negative, and supplies a negative potentialdetection signal of active level to the bias start-up circuit 18.

In this way, also when a control signal of active level as well as anegative potential detection signal of active level are sent to the biasstart-up circuit 18, in case the potential of the ripple terminal 14 isof inactive level, the bias start-up circuit 18 determines that theinput of the negative potential detection signal is invalid to set thebias start-up signal off. Accordingly, the bias circuit 19 is notstarted up. As a result, the power amplifier IC 10 is not supplied witha bias voltage, and stays in off state. In this case, in the push sideoutput transistor PT, the voltage between the drain and source is setapproximately to the voltage of the battery 21, which does not start upthe bias circuit 19. Therefore, it becomes possible to surely preventthe breakdown of the push side output transistor PT.

On the other hand, in case the power amplifier IC 10 is correctlyconnected to the power supply, a control signal of active level issupplied to the bias start-up circuit 18. Accordingly, the bias start-upsignal is set on, starting up the bias circuit 19. Furthermore, in casethe potential of the ripple terminal 14 rises to or more than apredetermined value, the potential of the ripple terminal 14 is appliedto the bias start-up circuit 18 through the ripple terminal potentialdetection circuit 20 as a signal of active level. Then, in the state inwhich the loudspeaker is driven, in case the negative potentialdetection circuit 17 is made to malfunction due to the backelectromotive force of the loudspeaker and a negative potentialdetection signal of active level is sent to the bias start-up circuit18, the bias start-up circuit 18 does not set the bias start-up signaloff. Accordingly, the push side output transistor PT is not cut off, andthe sound quality can be prevented from being deteriorated.

On the other hand, in case a control signal output from the control IC22 is of inactive level, the bias start-up circuit 18 of the poweramplifier IC 10 is off, and the bias start-up signal is off.

Table 1 shows above-described operations.

TABLE 1 Control Negative potential Potential of Bias start-up signaldetection signal ripple terminal signal Active level Active levelInactive level Off Active level Active level Active level On Activelevel Inactive level Active level On Inactive level Active levelInactive level Off Inactive level Inactive level Inactive level Off

(First Example of Negative Potential Detection Circuit)

FIG. 2 shows a circuit diagram of the first example of the negativepotential detection circuit 17 shown in FIG. 1. The negative potentialdetection circuit 17 includes an NPN type detection transistor Q1, aresistor R1 connected between the base of the transistor Q1 and theground terminal (GND terminal) 12 of the power amplifier IC 10, aresistor R2 connected between the emitter of the transistor Q1 and theoutput terminal (Out terminal) 13 of the power amplifier IC 10, and aresistor R3 connected between the collector of the transistor Q1 and thepower terminal 11 of the power amplifier IC 10. The collector of thetransistor Q1 is a detection output node A.

In case the power amplifier IC 10 is correctly connected to the powersupply, the potential of the output terminal 13 is higher than that ofthe GND, and a current does not flow through the base of the detectiontransistor Q1. Accordingly, the detection output node A becomes “H”level, indicating non-detection state.

On the other hand, in case the power amplifier IC 10 is incorrectlyconnected to the power supply, the potential of the output terminal 13becomes lower than that of the GND. In this case, when a forwarddirection voltage VBE occurs between the base and emitter of thedetection transistor Q1, a current flows through the base of thedetection transistor Q1. Accordingly, the detection transistor Q1 isturned on, and the detection output node A becomes “L” level, indicatingdetection state.

The resistors R1, R2 can have their values arbitrarily set up. Theresistors R1, R2 are arranged so as to improve the breakdown voltage fora surge input etc. in case a current path between the ground terminal 12and the output terminal 13 is formed at the time of the incorrectconnection state. One of the resistors R1, R2 may be removed.Furthermore, in case the emitter back withstand voltage of the detectiontransistor Q1 is low, as shown by a dotted line in FIG. 2, a PN junctionelement, whose direction is opposite to that of the base-emitterjunction (diode), may be connected in parallel between the base andemitter of the detection transistor Q1.

(Second Example of Negative Potential Detection Circuit)

FIG. 3 shows a circuit diagram of the second example of the negativepotential detection circuit 17 shown in FIG. 1. In the negativepotential detection circuit 17, the base of an NPN type transistor Q1for detecting ground potential is connected to a reference potentialVref, a resistor R1 is connected between the collector of the transistorQ1 and the power supply node, and a resistor R3 is connected between theemitter of the transistor Q1 and the ground potential GND.

The base of an NPN type transistor Q2 for detecting an output potentialis connected to the reference potential Vref, a resistor R2 is connectedbetween the collector of the transistor Q2 and the power supply node,and a resistor R4 is connected between the emitter of the transistor Q2and the output terminal 13 of the power amplifier IC 10.

The collector of the ground potential detection transistor Q1 and thecollector of the output potential detection transistor Q2 are connectedto two input nodes of a voltage comparator CP. The output node of thevoltage comparator CP is a detection output node A, which is connectedto the bias start-up circuit 18 shown in FIG. 1.

In case the power amplifier IC 10 is correctly connected to the powersupply, the potential of the output terminal 13 is equal to or higherthan the ground potential. In this case, by setting up the resistanceratio of the resistors R1, R2, R3, R4 so that the voltage drop of theresistor R2 is smaller than the voltage drop of the resistor R1, theoutput node A of the comparator CP becomes “L” level (non-detectionstate).

On the other hand, in case the power amplifier IC 10 is incorrectlyconnected to the power supply, the potential of the output terminal 13of the power amplifier IC 10 becomes lower than the ground potential. Inthis case, by setting up the resistance ratio of the resistors R1, R2,R3, R4 so that the voltage drop of the resistor R2 is larger than thevoltage drop of the resistor R1, the output node A of the comparator CPbecomes “H” level (detection state).

The resistor R4 has to have its value arbitrarily set up so as toimprove the breakdown voltage for a surge input, etc., of the outputterminal 13. Furthermore, in case the emitter back withstand voltage ofthe detection transistor Q2 is low, as shown by a dotted line in FIG. 3,a PN junction element, whose direction is opposite to that of thebase-emitter junction, may be connected in parallel between the base andemitter of the detection transistor Q2.

(Example of Bias Start-Up Circuit)

FIG. 4 shows a circuit diagram of an example of the bias start-upcircuit 18, bias circuit 19, and ripple terminal potential detectioncircuit 20 shown in FIG. 1. In the bias start-up circuit 18, a negativepotential detection node A to which a negative potential detectionsignal is supplied is connected to the ground potential GND throughserially connected resistors R2, R3. The connection node of theresistors R2, R3 is connected to the base of an NPN type transistor Q2through a resistor R4. The transistor Q2 has its emitter connected tothe ground potential, and has its collector connected to the groundpotential through a diode stack circuit in which diodes D1, D2 areserially connected. Furthermore, the transistor Q2 has its collectorconnected to the emitter of an NPN type transistor Q3 through a resistorR5. The transistor Q3 has its collector connected to the power (Vcc)node, and has its base connected to the control terminal 15.Furthermore, the transistor Q2 has its collector connected to the baseof an NPN type transistor Q4. This transistor Q4 has its emitterconnected to the ground potential through a resistor R7, and has itscollector connected to the power node through a resistor R6.

In the bias circuit 19, a PMOS type transistor M1 has its gate connectedto the collector of the transistor Q4 of the bias start-up circuit 18,and has its source connected to the power node. The transistor M1 hasits drain connected to the base of an NPN type transistor Q5 as well asto the emitter of the transistor Q5 through a resistor R8. Thistransistor Q5 has its collector connected to the power node, and has itsemitter connected to the ground potential through serially connectedresistors R9, R10, and diode D3.

The connection node of the resistors R9, R10 is connected to the rippleterminal 14 through a resistor R11 of the ripple terminal potentialdetection circuit 20. The capacitor 23 connected to the resistor R11 andto the ripple terminal 14 configures a ripple filter. In the rippleterminal potential detection circuit 20, an NPN type transistor Q6 hasits base connected to the ripple terminal 14, and has its collectorconnected to the power node, has its emitter connected to the groundpotential through serially connected resistors R12, R13. The connectionnode of the resistors R12, R13 is connected to the base of the NPN typetransistor Q1 through the resistor R1 of the bias start-up circuit 18.The transistor Q1 has its emitter connected to the ground potential, andhas its collector connected to the connection node of the resistors R2,R3.

In the bias circuit 19, an NPN type transistor Q7 has its collectorconnected to the power node, and has its base connected to the base ofthe transistor Q6. Furthermore, the transistor Q7 has its emitterconnected to the ground potential through a serially connected resistorR14 and diodes D4, D5. The diodes D4 and D5 constitute a diode stackcircuit. The connection node of the resistor R14 and the diode D4 isconnected to the base of an NPN type transistor Q8. This transistor Q8has its emitter connected to the ground potential through a resistorR15, and has its collector being a power amplifier bias node B connectedto the power amplifier driver circuit 16 a.

(Initial State)

The operation of the circuit shown in FIG. 4 which has above-describedconfiguration will be explained. In the initial state, the controlterminal 15 is of inactive level (“L” level), and the transistors Q3, Q4are in off state, and also the transistors M1, Q5 are in off state.Thus, the ripple filter capacitor 23 is not charged. Accordingly, alsothe transistors Q6, Q7 are in off state, and a power amplifier biascurrent does not flow through the transistor Q8.

(At the Time of Start-Up)

Next, the operation when the active level (“H” level) is applied to thecontrol terminal 15 from a microcomputer, etc., will be explained. Atthe moment when the control terminal 15 becomes “H” level, the ripplefilter capacitor 23 is not charged. Accordingly, the transistor Q6 staysin off state, and values of the resistors R12, R13 are previously set upso that also the transistor Q1 stays in off state in this state.

When the control terminal 15 becomes active level (“H”), the transistorQ3 comes into on state. At this time, in case a negative potentialdetection signal is of active level (“H”), the transistor Q2 comes intoon state, and it is possible to set the transistor Q4 in off state evenif the transistor Q3 is in on state. Owing to the start-up preventionoperation, even if the control terminal 15 becomes “H” level in the GNDopen and output grounding state, there is raised no possibility that abias current of a power amplifier flows through the transistor Q8.Accordingly, the power amplifier circuit 16 can be surely protected.

(Prevention of Malfunction in Correct Operation State)

When the power amplifier IC 10 is correctly connected to the powersupply, in case the control terminal 15 is of active level (“H”), and anegative potential detection signal is of inactive level (“L”), thetransistor Q2 is in off state, and the transistors Q3, Q4 are in onstate, and also the transistors M1, Q5 are in on state. Therefore, theripple filter capacitor 23 is charged, and a bias current of a poweramplifier flows through the transistor Q8. After the power amplifiertransfers to the bias state, the transistors Q6, Q1 are in on state. Asa result, even if a negative potential detection signal becomes activelevel (“H”), there is raised no possibility that the transistor Q2 comesinto on state. Accordingly, the operation of the bias circuit 19 is notprevented. Thus, when loudspeaker is driven by an output of the poweramplifier circuit 16, even if the negative potential detection circuit17 is made to malfunction due to the back electromotive force of theloudspeaker, the operation of the power amplifier circuit 16 is not cutoff. As a result, deterioration of the sound quality can be prevented.Accordingly, the negative potential detection circuit 17 and the biasstart-up circuit 18 can be of simple configuration that can be easilydesigned, securing the sound quality as well as the protection intensityat the time of the GND open and output grounding state of a poweramplifier system.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A power amplifier system comprising: a power terminal; a groundterminal; an output terminal; a ripple terminal; a control terminal towhich a control signal is supplied from outside; a power amplifiercircuit connected between the power terminal and the ground terminal, anoutput node of the power amplifier being connected to the outputterminal; a negative potential detection circuit connected to the outputterminal, detecting a negative potential of the potential of the outputterminal; a bias circuit which supplies a bias voltage to the poweramplifier circuit, the bias circuit having its output node foroutputting the bias voltage connected to the ripple terminal; and a biasstart-up circuit to which the control signal supplied to the controlterminal, a detection signal supplied from the negative potentialdetection circuit, and the potential of the ripple terminal aresupplied, the bias start-up circuit controlling the startup operation ofthe bias circuit.
 2. The system according to claim 1, wherein the poweramplifier circuit has first and second MOS transistors configuring apush-pull circuit, the first MOS transistor configures a push sideoutput transistor, and the second MOS transistor configures a pull sideoutput transistor, and a parasitic diode is located between the drainand source of the pull side output transistor.
 3. The system accordingto claim 1, wherein the bias start-up circuit comprises: a first circuitfor outputting a first signal to set the bias circuit in non-start-upstate based on the logical level of inactive state of the controlterminal in the initial state, for outputting a first signal to set thebias circuit in non-start-up state even if the logical level of thecontrol terminal becomes active at the time of GND open and outputgrounding state in which the ground terminal is open when the groundpotential is connected to the output terminal, and for outputting afirst signal to set the bias circuit in non-start-up state even if thelogical level of the detection signal supplied from the negativepotential detection circuit becomes active; and a second circuit formaking the bias circuit transfer to start-up state in case the logicallevel of the detection signal supplied from the negative potentialdetection circuit is inactive when the power amplifier circuit is innormal operation state, and for keeping the bias circuit in start-upstate even if the logical level of the detection signal becomes active.4. The system according to claim 1, further comprising: a batteryconnected to the power terminal; a loudspeaker connected to the outputterminal; and a control integrated circuit for supplying the controlsignal to the control terminal, the control integrated circuit using thebattery as a power supply for operation.
 5. The system according toclaim 1, further comprising: a ripple terminal potential detectioncircuit for detecting the potential of the ripple terminal, the rippleterminal potential detection circuit being connected to the rippleterminal, a detection signal of the ripple terminal potential detectioncircuit being supplied to the bias start-up circuit.
 6. The systemaccording to claim 1, wherein the negative potential detection circuitcomprises: a first NPN transistor that has its collector connected tothe power terminal, and has its emitter connected to the outputterminal, and has its base connected to the ground terminal, the firstNPN transistor having its collector connected to the bias startupcircuit.
 7. The system according to claim 1, wherein the negativepotential detection circuit comprises: a second NPN transistor that hasits collector connected to the power terminal, and has its emitterconnected to the output terminal, and has its base connected to areference power supply; a third NPN transistor that has its collectorconnected to the power terminal, and has its emitter connected to theground terminal, and has its base connected to the reference powersupply; and a voltage comparator that has its first and second inputterminals connected to the collectors of the second and third NPNtransistors, and has its output terminal connected to the bias start-upcircuit.
 8. The system according to claim 1, wherein the bias start-upcircuit comprises: a fourth NPN transistor whose base is supplied withthe potential of the ripple terminal, and has its emitter connected tothe ground terminal; a fifth NPN transistor that has its base connectedto the collector of the fourth NPN transistor, which is supplied with adetection signal of the negative potential detection circuit, and hasits emitter connected to the ground terminal; a sixth NPN transistorwhose base is supplied with the control signal, and has its collectorconnected to the power terminal, and has its emitter connected to thecollector of the fifth NPN transistor; and a seventh NPN transistor thathas its base connected to the collector of the fifth NPN transistor, andhas its collector connected to the power terminal, and has its emitterconnected to the ground terminal.
 9. The system according to claim 8,wherein the bias circuit comprises: a MOS transistor that has its gateconnected to the collector of the seventh NPN transistor, and has itsone end of the current path connected to the power terminal; an eighthNPN transistor that has its base connected to the other end of thecurrent path of the MOS transistor, and has its collector connected tothe power terminal; and a first and a second resistors which areserially connected between the eighth NPN transistor and the groundterminal, the first and second resistors outputting the bias voltagefrom the connection node thereof.
 10. The system according to claim 9,further comprising a ripple terminal potential detection circuitcomprises: a ninth NPN transistor that has its base connected to theripple terminal, and has its collector connected to the power terminal;and a third and a fourth resistors which are connected between theemitter of the ninth NPN transistor and the ground terminal, the thirdand fourth resistors outputting the potential of the ripple terminalfrom the connection node thereof.
 11. The system according to claim 10,wherein the ripple terminal is connected to a capacitor for ripplefilter.